The present invention relates to a television signal transmitting apparatus and, particularly, such apparatus capable of minimizing "dot crawl or cross-luminance phenomena"occurring in a boundary portion of a monochromatic character bearing, colored contour on a television receiver screen when the latter receives an NTSC television signal superposed with such monochromatic character with such colored contour by means of the so-called color-super imposing device.
The superposition of monochromatic characters with colored background on an NTSC color signal, referred to as "color letter super system", has been achieved by a circuit shown in FIG. 5. In FIG. 5, an original NTSC signal is input to a terminal 1 and a character signal from a monochrome caption camera or the like is input to a terminal 2. The character signal is passed through a slice circuit 3 to a contour signal generating portion 4 in which a contour signal is generated by expanding horizontal and vertical widths of the sliced character signal. The contour signal thus obtained is passed through a slice circuit 5 to obtain a key signal corresponding to a coloring area of the character and the key signal is supplied to one input of a gate circuit 7.
A 3.58 MHz color sub-carrier from a color sub-carrier oscillator 6 is supplied to an adder 8 set up. An output of the adder 8 is supplied to another input of the gate circuit 7. Thus, the key signal is gated with the output of the adder 8 and the coloring area is defined. The coloring area thus defined is supplied to an input of another gate circuit 9.
The gate circuit 9 has a second input to which the gated character signal from the slice circuit 3 is supplied and a third input connected to the terminal 1. The coloring area is set on the NTSC signal and the character signal is superposed in the coloring area in the gate circuit 9. A resultant signal is output at a terminal 10 of the gate circuit 9.
When monochromatic characters are to be superposed on a colored area by means of this color super imposing device, the 3.58 MHz signal is sharply cut-off in a boundary portion of the colored area, so that a frequency band width of the color signal in the boundary portion exceeds a range defined according to NTSC system.
Further, when the characters superposed by this circuit construction is received by a usual television receiver, a luminance/chrominance signal separation circuit of the receiver can not operate normally due to such wide frequency band width, resulting in that the 3.58 MHz signal in the boundary portion is mixed up to the luminance signal causing flicker effect, referred to as "dot crawl phenomenon", to be produced.
In order to remove such flicker effect, a motion adaptive three-dimensional luminance/chrominance signal separation circuit is provided in an EDTV receiver. When there is such 3.58 MHz signal mixed in the luminance signal, the three-dimensional luminance/chrominance signal separation circuit determines that luminance signal as "moving signal" and switches its construction to a two-dimensional luminance/chrominance signal separation circuit. Although this scheme is effective to remove "cross color"signal component, an image quality itself is degraded due to the fact that the three-dimensional circuit is inoperative.